A conventional hydrodynamic bearing system essentially includes a bearing sleeve, a shaft accommodated in a cylindrical bore of the bearing sleeve and at least one radial bearing section provided between the bearing sleeve and the shaft with the aid of which the shaft and the bearing sleeve are supported with respect to each other during operation of the spindle motor. A bearing gap formed between the shaft and the bearing sleeve is filled with a liquid lubricant, preferably bearing oil.
In general, hydrodynamic bearings are being increasingly used as rotary bearings of spindle motors for hard disk drives in place of roller bearings. Benefits of such hydrodynamic bearings, in comparison to rotary bearings which use roller bearings, include a low noise level, improved running precision and a significant increase in shock resistance. In addition, fewer parts are required for a hydrodynamic bearing assembly resulting in a considerable reduction in manufacturing costs.
In a hydrodynamic bearing, a preferably cylindrical shaft is rotatably supported within the bore of a bearing sleeve. The inner diameter of the bearing bore is slightly larger than the outer diameter of the shaft, so that a fine bearing gap is created between the opposing surfaces of the bore and the shaft. The bearing gap is filled with a lubricant, preferably bearing oil. In order to build up hydrodynamic pressure in the bearing gap, at least one of the opposing bearing surfaces is provided with a groove pattern. Due to the relative rotary movement between the opposing bearing surfaces, a pumping action is generated by the grooves within the lubricant that results in formation of a homogeneous lubricating film of regular thickness separating the bearing surfaces from each other. This homogenous lubricating film is stabilized by means of hydrodynamic pressure zones.
Since any contamination of data disks rotating within the clean-zone area of a hard disk drive inevitably results in sticking of the read/write head and total failure of the drive, it is necessary to protect the drive against leakage or splashing out of bearing oil from a hydrodynamic bearing. Such protection can only be provided by means of sealing methods which operate on a non-contact basis because contact-based sliding seats continuously generate foreign particles during rotation that can result in a head crash and can further lead to the total failure of the hard disk drive.
Protective or sealing effect of simple labyrinth seals or so-called “viscous seals” do not provide adequate protection in hydrodynamic bearing systems because they can not prevent oil from leaking and splashing out. Further, oil can penetrate through such seals into the disk area due to its propensity to seep.
U.S. Pat. No. 5,541,462 suggests the use of magnetic fluid seals in which, under the influence of a magnetic field, a continuous, annular film of ferrofluid is formed between the rotating and stationary bearing components. This involves a “liquid” seal confined in a magnetic field which can even withstand a certain difference in pressure. However, manufacturing the necessary ferrofluid is costly and the filling process is difficult and subject to errors. Thus, use of such ferromagnetic seals involves considerable extra cost. In addition, their use is limited to rotational speeds of up to 10 000 rpm since with higher rpms an additional flow loss increases greatly that reduces the overall efficiency of the motor to an unacceptable extent.
A much more reasonable solution in terms of costs, that does not cause additional loss even at high rpms, is based on the exploitation of the material-specific characteristics of the lubricant, in other words, it uses active principles behind capillary, adhesive and cohesive forces.
A solution in this respect was suggested, for example, in U.S. Pat. No. 5,667,309 in which the bearing bore features a tapered area at one end of the bearing sleeve in the shape of a conical counterbore, while the opposite end features an air-tight seal. The bag section shape of this hydrodynamic bearing increases lubricant's retention capability, particularly when subjected to shock, by means of which the sealing action of the seal, referred to as a “capillary seal”, is improved. Through the counterbore in the bearing sleeve, a concentric tapered area widening outwards in the direction of the bearing sleeve end is formed between the sleeve's inner surface and the shaft's outer surface which is filled partially with bearing oil. Lubricating oil covers surfaces of the sleeve and the shaft forming a meniscus with a concave contact surface between the oil and the air. Bearing oil held in the tapered area functions as a lubricant reservoir from which vaporized oil can be replaced. The tapered area between the inner surface of the cone and the outer surface of the shaft above the meniscus functions as an equalizing volume into which the bearing oil can rise when its temperature-sensitive volume increases with a rise in temperature causing the fluid level to increase. A bearing arrangement with a similar capillary seal is disclosed in DE 696 15 098 T2. Here, alongside the tapered counterbore at the end of the bearing sleeve, additional ducts are provided within the bearing sleeve to hold the lubricant.
The cohesive forces which are active in the fluid of the lubricant, supported by capillary forces in the bearing gap, prevent liquid bearing oil from escaping from the bearing and leaking into the clean-zone area. The slimmer the design of the tapered transition area and the higher the viscosity of the lubricant, the more effective the sealing action of this arrangement. The main factor limiting the operating life of a spindle motor with a bearing arrangement having a tapered capillary seal is the decrease in the quantity of lubricant over time since, due to vapor pressure, a continual vaporizing process takes place. With the loss of lubricant, the risk of metal surfaces contacting each other when the motor starts up and stops is increased. This process may cause foreign particles to rub off, some of these particles being larger than the thickness of the bearing gap. This results in dry running and galling of the bearing surfaces leading to blockage of the spindle motor.
A further disadvantage of the capillary seal solution revealed in both '309 and '096 references is the limited lifespan of the bearing because only a part of the available volume of the tapered area can be used as a lubricant reservoir due to its two-fold function. Another disadvantage is that the effective useful length of the bearing is reduced due to the tapered area being aligned axially inwards. Since the axial length and angle of inclination of the tapered area are dependent on the total filling volume and the viscosity of the lubricant, the ratio of the length of the tapered area to the length of the bearing becomes increasingly less favorable the thinner the lubricant. However, the use of low viscosity bearing oil, particularly for portable applications such as laptops, is indispensable due to the low power loss this oil provides.
It is also disadvantageous that the positive effect of the capillary forces on the lubricant retention capability is reduced disproportionately as the cross-section increases in size. This means that if the fluid level increases due to a rise in operating temperature and the device is subjected to an axial shock at the same time, there is an increased risk that bearing oil leaks out and is thrown off due to the reduced retention capability.
An important criterion for the suitability of hard disk drives having spindle motors with hydrodynamic bearings for use in portable devices is that power loss is kept to a minimum. As mentioned earlier, this objective can be achieved by using a low viscosity lubricant. However, the overall length of a capillary seal, of the art described above, which has been adapted for lower viscosity applications would increase to such an extent that the remaining useful bearing length would no longer suffice to accommodate a hydrodynamic radial bearing with a sufficient stiffness.
Other hydrodynamic bearing systems are known from U.S. Pat. No. 5,555,435 A and European Patent Publication EP 0 844 408 A2 in which a bore is provided in the bearing sleeve to ventilate the bearing gap or a lubricant reservoir. This bore, however, does not function as a lubricant reservoir or an equalizing volume.
German Patent No. DE15 25 198 A discloses a hydrodynamic journal bearing having an overflow duct for the lubricant between the radial bearing section and the axial bearing section. The duct is completely filled with lubricant and is not suitable for use as a lubricant reservoir or an equalizing volume.
U.S. Pat. No. 3,503,658A describes a hydrodynamic journal bearing system with a bearing sleeve sealed at one end, having an overflow duct for the lubricant provided between a pressure chamber formed on the sealed end of the bearing sleeve and the radial bearing section. Again this duct does not function as a reservoir or an equalizing volume.